The sensor is a fluorescence sensor, for example. In order to measure fluorescence, one generally irradiates the medium with a short-wavelength excitation light and detects the longer-wavelength fluorescent light produced by the medium. A fluorescence measurement is typically arranged as depicted in FIG. 1.
The fluorescence sensor 3 comprises a light source 1 and a receiver 2. The light source 1 transmits transmission light; the receiver 2 receives reception light. Since the fluorescent light is radiated in all spatial directions, the light paths of the transmission and reception light can in principle be at any angle α to each other.
Fluorescent and non-fluorescent particles scatter the transmission light. This scattered transmission light can also enter the receiver 2 and disturb the measurement of the fluorescence and lead to erroneous measurements.